1. Field of the Invention
The present invention relates generally to infrared filters, and more particularly to infrared filters having a laminated, layered structures and a method for the manufacture thereof.
2. Brief Description of the Prior Art
Far infrared filters have been previously made by vacuum deposition of thick films upon suitable substrates. However, the individual films are typically so thick that intrinsic mechanical stresses cause many failures. Such filters may take three weeks to produce and have only a 30% chance of producing a successful product. Therefore they are expensive, typically costing twenty thousand dollars for a single one inch diameter filter at a wavelength of approximately twenty-five microns.
U.S. patent application 3,551,017 entitled "Far Infrared Transmission Type Interference Filter", issued Dec. 29, 1970 to Toshikatsu Iwasaki, et al., discloses an interference filter having a plurality of layers which are arranged into two sets of layers, (F.A.L.) and (S.A.L.). Each of the layers in the F.A.L. set has an identical optical thickness, and each of the layers in the second set has an identical optical thickness, although the optical thickness of the second set of layers differs from the optical thickness of the first set of layers.
Other prior art known to the inventor includes U.S. Pat. No. 2,660,925, Light Reflector Which Transmits Infrared Rays, issued Dec. 1, 1953 to A. F. Turner which discloses a reflector of e.g. glass coated with germanium, which transmits a relatively large proportion of incident infrared light. U.S. Pat. No. 3,000,375, Semi-conductor Heat Absorption Means, issued Sep. 19, 1961 to M. J. E. Golay which discloses an infrared filter comprising a semiconductor such as germanium or silicon on a base of rock salt. U.S. Pat. No. 3,188,513, Optical Filters And Lamps Embodying The Same, issued Jun. 8, 1965 to R. L. Hansler which discloses an infrared filter having a film of e.g. silicon or germanium coated with a dielectric film. U.S. Pat. No. 3,331,941, Infrared Heater, issued Jul. 18, 1967 to J. W. Edwards et al. which discloses an infrared filter comprising alternate layers of germanium and cryolite on a quartz substrate. U.S. Pat. No. 3,718,533, Composite Sheets For Agricultural Use, issued Feb. 27, 1973 to S. Shibata which discloses a filter for agricultural use employing layers of polypropylene and aluminum. U.S. Pat. No. 4,245,217, Passive Infrared Alarm System, issued Jan. 13, 1981 to P. W. Steinhage which discloses an infrared filter having germanium and dielectric layers. U.S. Pat. No. 4,284,323, Trichroic Mirror, issued Aug. 18, 1981 to G. Jankowitz which discloses a germanium and silicon monoxide infrared filter on a quartz substrate.